The present invention relates to a directional transmission method and directivity control type communication apparatus.
A cellular system is adopted in digital radio communication systems such as portable telephones and car telephones. In the cellular system, a communication area is divided into cells each provided with a base station, and a plurality of mobile stations concurrently communicate with the base station.
This cellular system has been rapidly spread recently, and users thereof have been also rapidly increased, resulting in the important subject that the channel capacity in a cell should be increased.
One of techniques to increase the channel capacity is an adaptive array antenna technique.
In the adaptive array antenna technique, an output from each antenna of an array antenna apparatus comprised of a plurality of antennas is added an amplitude shift and phase shift, and then combined, thereby forming directivities for transmission and reception.
In other words, a weight to be multiplied each antenna output is determined based on a predetermined algorithm, and each weight is varied corresponding to a variation of surrounding environments, thereby controlling the directivities for transmission and reception.
It is possible to attempt to increase the uplink channel capacity by applying the adaptive array technique to uplink channels, and canceling the interference from other cells to improve the reception quality. Concerning this technique, various reports are performed, for example, in academic societies.
Further, the study recently has started to apply the adaptive array technique to downlink channels.
Herein, the application of the adaptive array antenna to downlink channels means that a base station estimates a direction of arrival of the signal from a desired mobile station, and forms the transmission radiation pattern toward an estimated direction to transmit a signal.
The directional transmission with the adaptive array can decrease the transmission power from the base station with the channel quality maintained. Further, the interference to other mobile stations can be reduced. It is thereby possible to attempt to increase the downlink channel capacity.
The above-mentioned directional transmission is generally performed according to the procedures of analyzing a signal received under multipath phenomenon, determining a received level and received quality of the signal for each path, selecting a path such that an estimated propagation environment is the best, and transmitting a signal using the path.
However, it is sometimes difficult to select the optimal path actually. For example, in the case where carrier frequencies are different between transmission and reception, the case occurs that the best path for reception is not the best path for transmission.
Further the case occurs that the best path varies for an instant due to movement of a mobile station and change of propagation environment of radio signal. In such a case, the desirable effect cannot be obtained even if the directional transmission is performed using a path selected at the time of reception as a path for transmission.
The present invention is carried out based on the above-mentioned investigation. An object of the present invention is to enable accurate selection of a path that is optimal for transmission to achieve effective directional transmission.
In the present invention, a path for use in transmission is selected in consideration of information indicative of a reception condition at a communication partner indicating how a signal transmitted from this side is received at the communication partner, in addition to information concerning reception conditions of signals transmitted from the communication partner through a plurality of paths. It is thereby possible to select a more accurate transmission path.
In one preferred mode of a transmission radiation pattern forming method of the present invention, the transmission radiation pattern is determined by referring to information indicative of a previous reception condition in a communication apparatus of the communication partner, which was transmitted from the communication apparatus of the communication partner previously.
For example, a current transmission radiation pattern is changed when the previous reception condition in the communication apparatus of the communication partner is not desirable. The information that the previous reception condition in the communication apparatus of the communication partner is not desirable means that a transmission path currently selected may not be optimal. Accordingly, changing the transmission path to another selectable one increases the possibility that the reception condition in the communication partner is improved. Further, in the case where it becomes difficult to judge the selection of transmission path only using the reception condition of the signal transmitted from the communication partner, it is possible to select a desirable transmission path accurately.
In another preferred mode of the present invention, a transmit power control command (TPC) returned from the partner is monitored, and in the case where there is a plurality of paths in which received signal levels and received signal qualities are almost equal, the switching of transmission radiation pattern is performed based on the history of transmit power control command (TPC). In this case, since the information of the transmit power control command (TPC) is utilized to select the transmission path, it is not necessary to use a particular control signal, therefor facilitating the achievement.
Further, a communication apparatus of the present invention switches the transmission path referring to the information on the previous reception condition in the communication device of the communication partner, and adaptively controls the transmission radiation pattern. Using this communication apparatus as a base station in a CDMA communication enables the base station to decrease the transmit power with the communication quality maintained. It is thereby possible to reduce the interference degree that currently transmitted radio signals provide to other mobile stations. Accordingly it is possible to increase the downlink channel capacity.